Be $S^1$ the unit circle in the plane, that is, $S^1= \lbrace (x,y) : x^2+y^2=1 \rbrace$ with the subspace topology. Show that $S^1 - \lbrace (1,0)\rbrace$ is homeomorphic to the open interval $(0,1)$.
I have an idea. Using this theorem:
Let $f:(X, \tau_1) \rightarrow (Y, \tau_2)$ be a homeomorphism. Let $a \in X$, so that $X - \lbrace a \rbrace$ is a subspace of $X$ and has induced topology $\tau_3$. Also, $Y-\lbrace f(a)\rbrace$ is subspace of $Y$ and has induced topology $\tau_4$. Then $(X - \lbrace a \rbrace, \tau_3)$ is homeomorphic to $(Y-\lbrace f(a)\rbrace, \tau_4)$. (Morris, "Topology without tears", remark 4.3.6)
My idea is to make $X=S^1$ and $Y=(0,1) \cup ?$ so, with the respective topologies using the theorem, $S^1-\lbrace (1,0)\rbrace$ is homeomorphic to $(0,1)$. But I'm stuck in the part of defining $Y$ and find the homeomorphism $f$ between $X$ and $Y$.
PD. Sorry about my English, but it is not my native language.